FIG. 1 is a circuit diagram illustrating a configuration of a resonant-type multiple-output switching power source apparatus according to a related art. In this multiple-output switching power source apparatus, the primary side of a transformer T1 includes a full-wave rectifying circuit 2 to rectify an AC voltage from a commercial power source 1, a smoothing capacitor C3 connected between output terminals of the full-wave rectifying circuit 2, to smooth an output from the full-wave rectifying circuit 2, first and second switching elements Q1 and Q2 made of, for example, MOSFETs connected in series between both ends of the smoothing capacitor C3 and receiving a voltage across the smoothing capacitor C3 as a DC input voltage Vin, a control circuit 10 to control ON/OFF of the first and second switching elements Q1 and Q2, a voltage resonant capacitor Crv connected in parallel with the second switching element Q2, and a series resonant circuit connected to both ends of the voltage resonant capacitor Crv.
The series resonant circuit has a primary winding P1 (the number of turns of N1) of the transformer T1, a reactor Lr, and a current resonant capacitor Cri that are connected in series. The reactor Lr is, for example, a leakage inductance between the primary and secondary sides of the transformer T1.
The secondary side of the transformer T1 includes a first rectifying-smoothing circuit connected to a first secondary winding S1 (the number of turns of N2) wound to generate a voltage whose phase is opposite to the phase of a voltage generated by the primary winding P1 of the transformer T1 and a second rectifying-smoothing circuit connected to a second secondary winding S2 (the number of turns of N3) wound to generate a voltage whose phase is opposite to the phase of the voltage generated by the primary winding P1 of the transformer T1.
The first rectifying-smoothing circuit has a diode D1 and a smoothing capacitor C1, to rectify and smooth a voltage induced by the first secondary winding S1 of the transformer T1 and output a first output voltage Vo1 from a first output terminal. The second rectifying-smoothing circuit has a diode D2 and a smoothing capacitor C2, to rectify and smooth a voltage induced by the second secondary winding S2 of the transformer T1 and output a second output voltage V02 from a second output terminal.
The multiple-output switching power source apparatus also has a feedback circuit 5 to feed a signal corresponding to a voltage generated on the secondary side of the transformer T1 back to the primary side. An input side of the feedback circuit 5 is connected to the first output terminal. The feedback circuit 5 compares a voltage across the smoothing capacitor C1 with a predetermined reference voltage and feeds an error voltage as a voltage error signal back to the control circuit 10 on the primary side.
According to the voltage error signal from the feedback circuit 5, the control circuit 10 alternately turns on/off the first and second switching elements Q1 and Q2 to conduct PWM control in such a way as to keep the first output voltage Vo1 constant. Each gate of the first and second switching elements Q1 and Q2 receives, as a control signal, a voltage involving a dead time of about several hundreds of nanoseconds. This enables the first and second switching elements Q1 and Q2 to alternately turn on/off without the ON periods of the first and second switching elements Q1 and Q2 overlapping.
Operation of the multiple-output switching power source apparatus according to the related art having the above-mentioned configuration will be explained with reference to waveforms illustrated in FIG. 2.
In FIG. 2, VQ2ds is a drain-source voltage of the second switching element Q2, IQ1 is a current passing through a drain of the first switching element Q1, IQ2 is a current passing through a drain of the second switching element Q2, Icri is a current passing through the current resonant capacitor Cri, Vcri is a voltage across the current resonant capacitor Cri, ID1 is a current passing through the diode D1, VN2 is a voltage across the first secondary winding S1, and ID2 is a current passing through the diode D2.
The first output voltage Vo1 is controlled by the control circuit 10 that receives a voltage error signal fed back to the primary side from the first rectifying-smoothing circuit through the feedback circuit 5 and conducts the PWM control on the first switching element Q1. As mentioned above, the first and second switching elements Q1 and Q2 are alternately turned on/off according to control signals from the control circuit 10 with a dead time of about several hundreds of nanoseconds.
In an ON period (for example, from time t11 to t12) of the first switching element Q1, the current resonant capacitor Cri accumulates energy through an exciting inductance of the primary winding P1 of the transformer T1 and the reactor Lr (leakage inductance between the primary and secondary sides of the transformer T1).
In an ON period (for example, from time t12 to t14) of the second switching element Q2, the energy accumulated in the current resonant capacitor Cri causes the reactor Lr and current resonant capacitor Cri to pass a resonant current and send energy to the secondary side. The exciting energy of the exciting inductance of the primary winding P1 is reset.
More precisely, in the ON period of the second switching element Q2, the primary winding P1 receives a voltage that is obtained by dividing the voltage Vcri across the current resonant capacitor Cri by the exciting inductance of the primary winding P1 and the reactor Lr. When the voltage applied to the primary winding P1 reaches a level of (Vo1+Vf)×N1/N2, the voltage is clamped and the current resonant capacitor Cri and reactor Lr pass a resonant current and send energy to the secondary side. This results in passing the current ID1 through the diode D1. When the voltage of the primary winding P1 is smaller than the level of (Vo1+Vf)×N1/N2, no energy is sent to the secondary side of the transformer T1 and the exciting inductance of the primary winding P1 of the transformer T1, the reactor Lr, and the current resonant capacitor Cri produce a resonant operation only on the primary side.
The ON period of the second switching element Q2 is determined by the ON period of the first switching element Q1 under a fixed frequency, or is an optional constant period. The ON period of the first switching element Q1 may be changed to change duty ratios of the first and second switching elements Q1 and Q2, thereby changing an energy quantity sent to the secondary side.
The first and second secondary windings S1 and S2 are coupled with each other at the same polarity. In an ON period of the second switching element Q2, energy from the first secondary winding S1 is outputted as the first output voltage Vo1. During this period, energy from the second secondary winding S2 is outputted as the second output voltage V02, which is substantially equal to a level of Vo1×N3/N2.
In practice, however, voltages generated by the first and second secondary windings S1 and S2 are higher than the first and second output voltages Vo1 and Vo2 each by a forward voltage drop Vf of the diodes D1 and D2. Accordingly, a change in Vf due to a change in load on each output worsens a cross regulation. In a power source apparatus with variable output voltages, a change in one output voltage results in proportionally changing the other output voltage. This makes it impossible to directly provide a plurality of outputs from windings.
FIG. 3 is a circuit diagram illustrating a configuration of a multiple-output switching power source apparatus according to another related art. This multiple-output switching power source apparatus employs, instead of the second rectifying-smoothing circuit illustrated in FIG. 1, a regulator 12 such as a dropper or a step-down chopper, to generate a second output voltage V02 from a first output voltage Vo1 so as to stabilize the outputs. This multiple-output switching power source apparatus may solve the cross regulation problem between two outputs. The regulator 12, however, increases a loss and additional parts such as switching elements, choke coils, and control ICs increase costs and packaging spaces. In addition, the switching regulator such as a step-down chopper unavoidably generates noise.
A multiple-output switching power source apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-259644 proposes a switching converter circuit that stabilizes two kinds of voltage with a single converter. This switching converter circuit employs a second switching element as an active snubber to control ON/OFF of a first switching element and stabilize a first output. During an OFF period of the first switching element, the circuit controls ON/OFF of the second switching element to stabilize a second output. This switching converter circuit may stabilize two kinds of output with a single converter. This circuit, however, must have two secondary windings because a secondary winding to provide the first output must have an opposite polarity with respect to a secondary winding that provides the second output.